1. Field of the Invention
This invention relates to high power optical apparatus employing large-mode-area (LMA), multimode, gain-producing optical fibers, and, more particularly, to high power optical amplifiers and lasers using such fibers.
2. Discussion of the Related Art
In order to amplify an optical signal propagating in the core of an optical fiber, typically the core is doped with a gain-producing species and then pumped by an optical pump at a wavelength that is absorbed by the species. In silica optical fibers the core is illustratively doped with a rare-earth element (e.g., Er, Yb, Er—Yb, Nd, Tm, Ho, etc) or chromium (Cr), which enables signals at wavelengths in the near infrared range (e.g., ˜1000-1600 nm) to be amplified. Er and Er—Yb silica fibers are commonly used to amplify signals at wavelengths above about 1500 nm and are typically pumped at or near either 980 nm or 1480 nm.
Applications of gain-producing fibers (GPFs) range from relatively low power applications such as telecommunications to much higher power applications such as materials processing, spectroscopy, and range finding. We direct our attention here to the latter.
Laser and amplifier systems based on GPFs are compact and rugged sources of high power radiation (e.g., >10 s of kW of peak power). For safety reasons it is desirable to operate such systems at eye-safe wavelengths greater than 1500 nm. Amplifiers employing well-known double clad fibers (DCFs) co-doped with Er—Yb are typically used at these wavelengths. They are pumped by an array of low brightness diode lasers. Because the optical output of the array has a large angular distribution, it can be coupled efficiently only into multimode fibers with large numerical apertures (NAs). To use the diode energy efficiently, the DCF geometry is utilized for GPFs, where the central core (in which the signal propagates) is doped with a gain-producing species and is surrounded by a high NA waveguide. The waveguide includes an undoped, inner cladding that surrounds that core and guides the diode array pump light and a lower refractive index outer cladding that surrounds the inner cladding. This double clad geometry reduces the pump absorption per unit length by a factor approximately proportional to the ratio of the inner cladding area to the core area. Therefore, a relatively high concentration of core dopant is required to achieve high pump absorption over a short fiber length, which is desirable to minimize nonlinear effects, amplified spontaneous emission (ASE), and signal re-absorption.
However, Er concentration in silica is limited by pair-induced quenching. Er—Yb co-doped fibers, where pump energy is absorbed by a high concentration of Yb and transferred to the Er, overcome this limitation and provide high pump absorption and gain per unit length. [See, A. Galvanauskas, “Mode-Scalable Fiber-Based Chirped Pulse Amplification Systems,” IEEE J. Selected Topics in Quant. Electr., Vol. 7, No. 4, pp. 504-517 (2001), which is incorporated herein by reference.] This approach has resulted in the generation of pulses with 262 μJ energy (before the onset of nonlinearities) but with a deteriorated beam with M2=2.1. [See, M. Savage-Leuchs, et al., “High pulse energy extraction with high peak power from short-pulse, eye safe all-fiber laser system,” Proc. of SPIE, Vol. 6102, pp. 610207-(1-8) (2006), which is also incorporated herein by reference.] However Er—Yb co-doped fibers have certain disadvantages. To achieve efficient energy transfer from Yb to Er they are co-doped with large amounts of phosphorus (P), which raises the core index, thereby limiting the maximum achievable mode-field-area (MFA) and making them highly multimoded. The refractive index profile of the core usually has a large center dip due to the burn-off of P during preform manufacture, which distorts the spatial mode. They are pumped at wavelengths between 900 nm and 1000 nm, and therefore the quantum efficiency for gain at 1500 nm is low. Significant heat is, therefore, generated and cooling may be required to prevent damage to the polymer coating that surrounds the inner cladding.
Alternatives to optical amplifiers predicated on the Er—Yb DCF design have been proposed. [See, for example, D. Taverner et al., “158-μJ pulses from a single-transverse-mode, large-mode-area erbium fiber amplifier,” Opt. Lett., Vol. 22, No. 6, pp. 378-380 (1997), which is incorporated herein by reference.] In this decade-old paper the authors describe an optical fiber amplifier in which the power stage included a LMA, single mode, Er-doped fiber was end pumped in a backward direction by a Ti-sapphire laser at 980 nm. A 1534 nm signal to be amplified was coupled through a preamplifier stage to the power stage via standard bulk optical components. The coupling optics between the Er-doped fiber and the preamplifier stage were chosen to accommodate their large NA mismatch. This design was reported to have amplified 10-100 pJ signal pulses to an energy of 158 μJ and peak powers of >100 kW. However, the Taverner amplifier design is disadvantageous for several reasons: (i) it uses multiple bulk optical components, which are difficult to align and to keep aligned as environmental conditions change with time; (ii) it uses a Ti-sapphire solid state pump laser, which is large, difficult to control, and has limited power; and (iii) it uses a single mode Er-doped fiber, which means that the MFA is limited and hence the energy storage capacity of the fiber is likewise limited.
Thus, there continues to be a need in the art for high power, GPF apparatus that alleviates one or more of the shortcomings of the prior art designs.